High-sulfidation (HS) epithermal deposits

Transcription

1 High-sulfidation (HS) epithermal deposits Zhaoshan Chang Noel White Jeff Hedenquist Dave Cooke Holiday and Cooke (2007) 12 December 2013, Hefei, China Photo Courtesy of Danilo Marcos, Gold Fields High-sulfidation (HS) epithermal deposits - Definition - Characteristics - Alteration: mineralogy and textures - Lithocap - Mineralization: mineralogy and textures : What is a HS deposit? Varied over time. Recently some proposed classification based on major sulphides! Mostly enargite luzonite, hypogene covellite - Einaudi, Hedenquist and Inan, 2003 Sulfidation states Covellite Digenite Enargite - Genesis - Exploration Cerro Yanacocha open pit, Peru

2 What is a HS deposit? Einaudi, Hedenquist and Inan, 2003 Einaudi, Hedenquist and Inan, 2003 Sulfidation states Sulfidation states What is a HS deposit? diagnostic features Not practical:! Ores: small volume, hard to find! Not helpful to exploration Covellite Digenite Enargite Alteration assemblage + ore mineral assemblage Alteration :! Silicic (vuggy quartz/residual quartz) pyrite /! Advanced argillic (AA) alteration: hypogene quartz alunite pyrite AA) Ore minerals: Enargite luzonite, hypogene covellite Cerro Yanacocha open pit, Peru Tantahuatay, Peru Epithermal gold deposits: production + reserves Location of principal epithermal gold deposits LS subtype Vi? Arribas et al. (2000)

3 Why explore for HS deposits? - Significant Au-Cu resources - Some Pb-Zn under right geological conditions - Potential for linked, deeper porphyry deposits Characteristics Alteration Silicic: >95 wt% SiO 2, residual/vuggy quartz, trace rutile, zircon >95 wt% SiO 2, / Advanced argillic: Quartz + pyrite + (alunite-aps and/or pyrophyllite) ± diaspore ± dickite ± kaolinite ± zunyite (F) ± topaz (F) ± cordierite (Mg) ± dumorterite (B) ± corundum ± andalusite + + (/ ) ±± ± ± ± ± ± ± ± Free gold in weathered HS deposit; Yanacocha, Peru APS: aluminium phosphate sulphate Hedenquist et al., 2000; Seedorf et al., 2005 Characteristics Alteration Silicic alteration Intermediate argillic: pyrite + (illite illite/smecitite or smectite) ± chlorite + ( / ) ± Argillic: kaolinite Propylitic: chlorite ± epidote ± actinolite ± albite ± calcite ± pyrite ± ± ± ± ± Typically vuggy ( vuggy quartz ), if the protolith has phenocrysts or clasts Matrix also changed; fine quartz now Pierina Summitville Different people use these terms in different ways read their definitions Hedenquist et al., 2000; Seedorf et al., 2005 Summitville Arribas et al., 2000

4 Silicic alteration Not always vuggy depends on the protolith Protolith = basalt Open the rock to examine Clay: dickite, kaolinite Vuggy texture? Baguio, Philippines AA alteration Al > Fe After Ms. HUANG Yunhui Alunite Fe > Al Alunite supergroup, include 3 groups, > 40 minerals Alunite group AluniteKAl 3 (SO 4 ) 2 (OH) 6 Natroalunite NaAl 3 (SO 4 ) 2 (OH) 6 Huangite CaAl 6 (SO 4 ) 4 (OH) 12 Jambor, 1999 Other minerals in AA alteration Kandite Group Kaolinite Al 2 Si 2 O 5 (OH) 4 Dickite Al 2 Si 2 O 5 (OH) 4 Halloysite Al 2 Si 2 O 5 (OH) 4 Nacrite Al 2 Si 2 O 5 (OH) 4 Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Diaspore AlO(OH) Topaz Al 2 SiO 4 (F,OH) 2 Zunyite Al 13 Si 5 O 20 (OH,F) 18 Cl Dumortierite Al BO 3 (SiO 4 ) 3 (O,OH) 3 Corundum Al 2 O 3 Andalusite Al 2 SiO 5 Cordierite Mg 2 Al 4 Si 5 O 18 Only Si and Al

5 SWIR spectral instruments PIMA PNIRS Original texture preserved Advanced argillic alteration various textures Alunite Quartz alunite Open space filling Vug-filling Alunite Alunite TerraSpec Advanced argillic alteration various textures and mineralogy Advanced argillic alteration various textures and mineralogy Wormy Alunite, pyrophyllite, diaspore, topaz Alunite Quartz Alunite Intergrowth of quartz and alunite Breccia Patchy wormy (Gusano)

6 AA alteration textures Vein Alunite Massive silicic Alteration Geometry Vertical Sub-horizontal Summiteville, USA by Arribas, 1995 Quartz latite porphyry Andesite Vertical; Faultcontrolled Silicic AA alteration - 500m Plumosealunite vein Lithocap Quartzalunite - Silicic AA alteration - Vein Lepanto, Philippines Lepanto 500m Lithocaps! Intense AA + silicic alteration +! Nearly horizontal Shuteen, Mongolia Lithocaps have structural roots the vertical part! Large size (up to >20 km 2 ) >20 km 2 Fresh Qtz-alun Vuggy dacite halo qtz ore Photo Courtesy of Danilo Marcos, Gold Fields quartz Tantahuatay, Peru

7 Lithocap not necessarily follow one stratigraphic unit Ï?gm _N C Occ& LITHOCAPS can be produced by ONE SOURCE, ONE EVENT OR MANY SOURCES, MANY EVENTS Permeability critical! ñũ 8ƀ ŌO Û - Zoning pattern of alteration -Vertical geometry PµÏ?( 6k u ptþ/ù ÙĊēń ēń-¾ ¾Ĕē ēń-¾ ¾Ĕē ÇŌ O ē }ķ åē }Ň Ŀē }) Ə ē?ē ºĕü Summitville Ĭb ptþ/ù ÙĊ ēń Summitville, USA; Steven & Ratté, 1960 Summitville Hedenquist, 2008 Ï?gm@ ± ±W W #v}

8 Zoning pattern of alteration - Lithocap / - Zoning pattern of alteration - Lithocap Dominantly AA alteration, minor silicic - Lack of horizontal mineralogical zonation - - Fluid flow changed - Mineralogical zonation normal to fluid flow direction! zoning vertical, not horizontal Dickite kaolinite - Quartz alunite - Dickite kaolinite - HS mineralisation - Ore minerals Lepanto, Philippines luzonite-cemented breccia Colquijirca, Peru Pueblo Viejo, Dominican Republic enargite and pyrite Colloform and brecciated sphalerite veins HS mineralisation - Ore minerals Most abundant: Pyrite FeS 2 Enargite Cu 3 AsS 4 luzonite Cu 3 AsS 4 Common: Tetrahedrite (Cu,Fe) 12 Sb 4 S 13 Tennantite (Cu,Fe) 12 As 4 S 13 Covellite Digenite CuS Cu 9 S 5 Electrum Au-Ag Marcasite FeS 2 Chalcopyrite CuFeS 2 Sphalerite ZnS Galena PbS Minor to trace: - Minor to trace: - Hessite Ag 2 Te Petzite Ag 3 AuTe 2 Native Te Te Chalcocite Cu 2 S Bornite Cu 5 FeS 4 Stannite Cu 2 FeSnS 4 Mawsonite Cu 6 Fe 2 SnS 8 Cassiterite SnO 2 Wolframite (Fe,Mn)WO 4 Stibnite Sb 2 S 3 Molybdenite MoS 2 Orpiment As 2 S 3 Realgar AsS Cinnabar HgS Iodyrite AgI Naumannite Ag 2 Se Umangite (?) Cu 3 Se 2 Klockmannite CuSe Colusite Cu 3 (Sn,V,As,Fe)S 4 Emplectite(?) CuBiS 2 Famatinite Cu 3 SbS 4 Native sulfur S Goldfieldite Cu 12 (Te,Sb,As) 4 S 13 Calaverite AuTe 2 Krennerite AuTe 2 Scorodite FeAsO 4 2H 2 O

9 Pb - Zn in HS ore Minor but ubiquitous Pb-Zn can be significant where there is carbonate 6.9% Zn, 1.3% Pb, 47 g/t Ag HS Mineralization ore textures Breccia common; enargitepyrite or covellite as cement Clasts silicified Quartz pyrophyllite diaspore -- Colloform sphalerite Enargite-pyrite - Pueblo Viejo, Dominican Republic Tantahuatay, Peru Covellite-dominant Zijinshan Tantahuatay, Peru Breccia ore HS ore textures Vein Vug-filling Disseminated Enargite Enargite Enargite luzonite pyrite - - Yanacocha, Peru Tantahuatay, Peru Lepanto, Philippines Silicic clast Enargite-pyrite - Tantahuatay, Peru

10 Ore stage alteration silicification HS orebodies Mostly in structure roots, e.g., Lepanto, ~70% in roots, ~30% in the cap part Lepanto, ~70% ~30% quartz Lepanto, Philippines Lepanto Sillitoe, 1993; Hedenquist et al., 2000 Alteration size vs. resources Position of mineralization in lithocap HS mineralization occurs relatively close to intrusive centre Causative intrusions may have porphyry-style alteration El Indio 8 Moz (Arribas et al., Gold in 2000 Slide Courtesy J Hedenquist) Lepanto, Philippines

11 Genesis / - Alteration conditions - Info from the mineralogy Ore textures indicate a 2- stage process: early alteration and late mineralization T = 250 C 1. Silicic alteration needs to get Al removed 1. Significant dissolution of alunite + kaolinite (>1000ppm total Al in solution) occurs at ph<2 ph < 2 Needs lower ph if there is only alunite Tantahuatay, Peru Summitville: Stoffregen, quartz Quartz pyrite - Pyrite dissolution Significant alu-kao dissolution Alunite Quartz-alunitepyrite AA alteration 2. Quartz - alunite - pyrite stable conditions (Ib): ph: 2 to ph: 2 to Vuggy quartz pyrite stables Region Ia -Ia Alunite? T Alteration conditions - Info from the mineralogy ph T conditions Residual quartz APS Alunite Corundum (?) Diaspore (?) Andalusite Pyrophyllite Dickite? Kaolinite ph Mg: Cordierite F: Topaz, Zunyite B: Dumortierite Summitville: Stoffregen, 1987 Seedorff et al., 2005

12 Alteration conditions Info from the mineralogy What kind of environments can produce this acidity? O RÊ Ü âq Ï? # ˺ ÅB +{ Reyes, 1990; Hedenquist et al., 1996, 2000 Hedenquist, 2008 White Island, New Zealand: High to low T fumaroles Ð/White Island f ò I' K \ Hypogene, up to 850 C vapors, with HCl, SO2, CO2, H2S ï Þ- ƏŢ850 C Y HCl, SO2, CO2, H2S Steam-heated, ~100 C, only CO2, H2S ňâaùčþ- ~100 C, QY CO2, H2S Hedenquist, 2008 Outflow of acid (ph 0.6) hypogene condensate ï Þ-;< čŭ õ í(ph 0.6)o oé Hedenquist, 2008 White Island, New Zealand t Au, 1 Mt Cu flux to atmosphere over life of ~10,000 yrs ő6white Island - cqį1 ŭw qþ ³"į300X Xů 1Č Č XŹ

13 Satsuma Iwojima, S. Kyushu: passive degassing (~1000 yrs) Satsuma Iwojima Kawah Ijen, Java: volcanic crater with acidic lake Kawah Ijen ph 0.0 Condensation of acid magmatic volatiles Hedenquist, 2008 Hedenquist, 2008 Photograph: Pierre Delmelle Magmatic vapor GAS PHASE low density high SO 2, HCl SO 2, HCl low NaCl, metals Evolution of magmatic fluids Single phase Magmatic fluid At shallow depths (< 4 km) aqueous magmatic fluids split into two separate phases (< 4 km) Partitioning changes with changes in P, T LIQUID PHASE higher density low SO 2, HCl SO 2, HCl high NaCl, metals V L H+L 80% 20% L+V H+V L+H+V

14 Vapor condensates, then dissociates Lower T, more dissociation, more acidic Zoning pattern of alteration - Vertical geometry / - HCl = H + + Cl - - SO 2 disproportionation SO 2 : 4SO 2 + 4H 2 O = 3H 2 SO 4 + H 2 S H 2 SO 4 = 2H + + SO 4 2- H 2 S = 2H + + S 2- Acid leaching Sulphate, e.g., alunite Pyrite ph ~ >5 <2 Summitville, USA; Steven & Ratté, 1960 / Vuggy qtz zone flares upward due to cooling, increased reactivity HCl,SO 2 Varying intervals between base of HS deposit and top of porphyry Petevolo Mineralization in high-sulfidation epithermal deposits - Hypotheses Acidification of meteroric component dominant near neutral ph chloride water that invades the acid leached zone - Stoffregen (1987), Berger and Henley (1989) Cooling and dilution of rising magmatic brine - White (1991), Hedenquist et al. (1994) Cooling and dilution of low-salinity magmatic fluids that come from deep-seated magma chamber as a single phase Shinohara and Hedenquist (1997), Hedenquist et al. (1998) Transpt. by vapor, followed by contraction into liquid, then metal dep. through mixing with meteoric water, de-s or boiling - Heinrich et al. (2004), Heinrich (2005), Williams-Jones and Heinrich (2095)

15 Exploration New advancement! Large size (up to >20 km 2 ), Intense alteration >20 km 2! No mineralogical, texture, or simple geochemical zoning pointing to ore or intrusive centre Shuteen, Mongolia Photo Courtesy of Danilo Marcos, Gold Fields Mankayan district, Philippines Mankayan Genetic relationship confirmed by dating; fluid inclusion study (O, H, S) - Arribas et al. (1995), Hedenquist et al. (1998) NW end of lithocap 4 km FSE porphyry Tantahuatay, Peru Chang et al. (2009, 2010) Buaki porphyry Lepanto HS:> 0.9 Mt Cu & 102 t Au FSE porphyry: % Cu & 0.6 g/t Au Alteration and Mineralization Exploration alunite SWIR spectral feature Victoria veins, g/t Au + Ag-Cu-Pb-Zn Teresa veins, g/t Au 1 km Nayak veins Mohong Hill porphyry + HS: Guinaoang porphyry, % Cu & 0.4 g/t Au SWIR: Short Wavelength Infra-Red

16 Alunite absorption peak at ~1480 nm shifts to higher position closer to intrusive centre 1480nm Alunite feature at ~1480 nm related to Na content 1480nm Na/(Na+K) (Stoffregen and Cygan, 1990) Higher Na/(Na+K) ratio indicates higher formation temperature (Stoffregen and Cygan, 1990) Alunite composition (LA-ICP-MS) (LA-ICP-MS) Sr/Pb La, Sr and La/Pb increases, and Pb decreases La, Sr La/PbPb Whole rock Cu<0.1%, Au<0.1ppm samples with alunite Cu<0.1%, Au<0.1ppm Sr/Pb La/Pb increases; Hg, Ag, Ag/Au, Te, As/Zn decrease La/PbHg, Pb/(Na+K), Ag, Ag/Au

17 Applying the above methods needs to distinguish AA alterations produced in other environments AA alteration could be: - Hypogene - Steam heated - Supergene Other methods: Elevation of lithocap patches at Mankayan Lepanto Focusing exploration at the lowest lithocap elevation at Mankayan would have found mineralised structures connected to Lepanto! Chang et al., 2011; Elevation contour of Imbanguila base from Garcia (1991) Other techniques Sample cliffs / massive quartz " Massive quartz good sign of mineralizing fluids " Distinguish from steam-heated silica cap High resistivity anomaly could indicate massive quartz zone Exploration combined methods Mapping! Geology! Alteration; use SWIR instruments; distinguish AA types! Structures / unconformities / Alunite spectral features Alunite geochemistry Whole rock geochemistry; filtered Geophysics; resistivity

18 Exploration combined methods Apply model! Spatial patterns! Fundamental processes + local conditions, e.g., wall rock